This invention relates to a gate drive circuit of an IGBT (Insulated Gate Bipolar Transistor which is an insulated gate self-extinction elements) of a switching semiconductor device and in particular to a gate drive circuit which enables suppression of a voltage surge during the turn-off operation and a decrease in a switching loss.
FIG. 18 is a block diagram to show a semiconductor device drive circuit described in Unexamined Japanese Patent Publication 5-328746, for example. In the figure, numeral 1 is an insulated gate bipolar transistor for controlling the conduction state between main electrodes in response to a voltage applied to a control electrode, numeral 1a is a control electrode, numeral 1b is a first main electrode, and numeral 1c is a second main electrode. For simplicity, the insulated gate bipolar transistor 1 will be hereinafter referred as the IGBT 1, the control electrode 1a called the gate 1a, the first main electrode 1b called the collector 1b, and the second main electrode 1c called the emitter 1c. Numeral 2 is an on-DC voltage source for supplying a voltage to turn on the IGBT 1, numeral 3 is an off-DC voltage source for supplying a voltage to turn off the IGBT 1, numeral 4 is switching signal generation means for generating a signal for turning on/off the IGBT 1, numeral 5 is on/off switch means for switching the on-DC voltage source 2 and the off-DC voltage source 3 in response to output of the switching signal generation means 4 for applying a voltage to the gate 1a of the IGBT 1, and numeral 6 is gate resistor switch means for switching gate resistors. The gate resistor switch means 6 is made up of a resistor 9, a resistor 10, and switch means 11. Numeral 7 is current detection means for detecting an electric current flowing into the IGBT 1 and numeral 8 is control means for controlling the gate resistor switch means 6 in response to output of the current detection means 7.
FIG. 19 is a flowchart to show a control method of the control means 8. In the control method, the control means 8 determines whether or not the conduction current is smaller than a reference current value based on output of the current detection means 7 for detecting the conduction current of the IGBT 1. If the control means 8 determines that the conduction current is smaller than the reference current value, it outputs a signal for switching the gate resistor so that the resistance value increases, and terminates the process. If the control means 8 does not determine that the conduction current is smaller than the reference current value, it does not output a signal for switching the gate resistor so that the resistance value increases, and terminates the process.
Next, the operation will be discussed with reference to FIGS. 18 to 21. When the switching signal generation means 4 outputs an on-signal, the on/off switch means 5 switches to the on-DC voltage source 2 for applying an on-voltage via the gate resistor switch means 6 to the gate 1a of the IGBT 1, turning on the IGBT 1. When the IGBT 1 is on, the current detection means 7 detects a conduction current of the IGBT 1 and the control means 8 determines whether or not the detected conduction current value is smaller than a given reference current value. If the control means 8 determines that the conduction current value is smaller than the reference current value, it supplies a resistor switch signal to the gate resistor switch means 6 for switching the switch means 11 so that the resistance value of the gate resistor increases. Specifically, the switch means 11 which is short-circuited is opened for using only the resistor 9 as the gate resistor. Next, when the switching signal generation means 4 outputs an off signal, the on/off switch means 5 switches to the off-DC voltage source 3 for applying an off-voltage via the gate resistor switch means 6 to the gate 1a of the IGBT 1, turning off the IGBT 1. At this time, since the switch means 11 is open, the turn-off operation is performed only with the resistor 9. If the control means 8 determines that the conduction current value is larger than the reference current value, the switch means 11 remains short-circuited and the resultant resistance of the resistors 9 and 10 is used as the gate resistor. Since the resistors 9 and 10 are connected in parallel, the resultant resistance becomes smaller than the resistance value of the resistor 9. Therefore, according to the operation, the gate resistance increases only if the conduction current value is smaller than the reference current value.
FIGS. 20A and 20B are waveform charts to show the effect of the gate resistance magnitude on the switching waveform when the IGBT 1 is turned off. It shows collector current Ic of conduction current of the IGBT 1, collector-to-emitter voltage Vce of the IGBT 1, and switching loss P of the IGBT 1. FIG. 20A shows the effect when the gate resistance is small and FIG. 20B shows the effect when the gate resistance is large. The larger the gate resistance, the more moderate the decrease rate of the collector current Ic when the IGBT is turned off. Thus, the collector-emitter voltage surge caused by applying the decrease rate of the collector current Ic to wiring inductance is decreased. However, the increase rate of the collector-to-emitter voltage Vce also becomes moderate at the same time, thus the switching loss P represented by the multiplication integral value of the collector current Ic and the collector-to-emitter voltage Vce is increased. Thus, the voltage surge can be suppressed by increasing the gate resistance value, but the switching loss is increased accordingly.
Here, the turn-off characteristic of the IGBT 1 will be considered. Fall time tf of the time between the collector current Ic starting to decrease and stopping decreasing when the IGBT 1 is turned off depends on the collector current Ic and has an increasing function characteristic with Ic, as shown in FIG. 21. In the large current region, the fall time tf is prolonged because of the device characteristic and the current decrease rate represented by (collector current/fall time) lessens as a logical consequence, thus the voltage surge does not cause a problem. In contrast, in the small current region, the fall time tf is shortened and the voltage surge increases, thus decrease countermeasures become necessary. Therefore, only when the conduction current is in the small current region, the gate resistance value is increased and the fall time tf is prolonged, whereby the voltage surge can be suppressed. At this time, the gate resistance value is lessened in the large current region, so that the switching loss is not increased.
The conventional semiconductor device drive circuit is thus configured and assumes the device characteristic of the IGBT with the fall time increasing with an increase in the collector current, thus can be applied only to specific devices. With the conventional semiconductor device drive circuit, the minimum value of the gate resistance is determined to suppress the voltage surge, thus the gate resistance cannot be lessened exceeding the limit of the voltage surge. As a result, measures of lessening the gate resistance for decreasing the switching loss cannot effectively be taken.